The patent No. PCT/WO 00/69565 in the name of G. Medoro describes an apparatus and a method for the manipulation of particles via the use of closed dielectrophoretic-potential cages. The method described teaches how to control the position of each particle independently of all the others in a two-dimensional space. The force used for entrapping the particles in suspension is negative dielectrophoresis. The individual control on the operations of manipulation is carried out by programming memory elements and circuits associated to each element of an array of electrodes integrated in one and the same substrate. There follows an important limitation due to the dimensions of each trap, limited by the need to integrate in the space corresponding to an individual electrode the electronics necessary for programming. Furthermore described in G. Medoro et al., 3, 317-325 (2003) IEEE Sensors Journal is an apparatus for the manipulation of cells based upon the use of parallel elongated electrodes, control of which does not require the use of transistors integrated in the substrate itself. The shape and spatial distribution of the elongated electrodes enables creation of traps of a cylindrical shape, by means of which it is possible to entrap groups of particles. There follows an important limitation due to the impossibility in the independent manipulation of individual particles.
Other methods for the manipulation of particles based upon dielectrophoresis do not enable independent control on a multiplicity of particles, as described by T. Schnelle et al., Biochim. Biophys. Acta 1157, 127-140 (1993). There follows an important limitation in the applications that require the study of the interaction between a multiplicity of cells.
Other methods based upon dielectrophoresis require direct contact between cells and substrate, since they make use of the force of positive dielectrophoresis (PDEP). In particular, described in J. Suchiro, J. Phys. D: Appl. Phys., 31, 3298-3305 (1998) is a method that envisages the creation of traps capable of attracting to the substrate a particle by means of forces of positive dielectrophoresis (PDEP). The particle consequently adheres to the substrate, from which it can be detached and pushed towards a new region by means of an appropriate distribution of force of negative dielectrophoresis (NDEP). In addition to the risk of causing irreparable damage to the cells, there follow some important limitations, such as for example the impossibility of using physiological solutions with high electrical conductivity or the impossibility of operating with polystyrene microspheres, since in both cases there do not exist the conditions necessary for activating the force of positive dielectrophoresis.
Likewise, the patent No. U.S. Pat. No. 6,294,063 in the name of Becker et al. describes a method and apparatus for the manipulation of packets of solid, liquid or gaseous biological material by means of a distribution of programmable forces. Also in this case the contact with a surface of reaction is a requisite indispensable for the operation of the method and apparatus. But the biggest limitation is linked to the need for a number of control signals (n×m) corresponding to the number of electrodes (n×m) if it is desired to use a passive substrate (and hence a less costly one). In order to increase the number of electrodes of the order of many hundreds or thousands it is necessary to use an active substrate, as explained in P. R. C. Gascoyne et al., Lab Chip, 2004, 4, 299-309, which includes transistors for addressing individually the n×m electrodes and generating locally the control signals. In this way, the number of input signals to the chip can be maintained within acceptable limits.
Another known method for the manipulation of liquid particles (droplets) is electro-wetting on dielectric (EWOD), described in T. B. Jones, Journal of Micromechanics and Microengineering, 15 (2005) 1184-1187. In this case, an electrical field exerted by electrodes made on a substrate enables the propulsion of a droplet surrounded by a gaseous phase in a direction controlled by the sequence of energized electrodes. Devices based upon this principle can be obtained by including a lid (also this coated with a dielectric), as is taught by the patent application No. US 2004/0058450A1 in the name of Pamula et al., or also simply a wire referred to as “chain”, which establishes the electrical contact with the droplets on top of the substrate J. Berthier et al., NSTI Nanotech 2005, www.nsti.org, vol. 1, 2005. In a way similar to what has been discussed above regarding the use of dielectrophoresis, in order to manipulate particles on a complete two-dimensional array via EWOD the embodiments reported in the known art resort either to a use of input signals corresponding to the number of electrodes of the array or to the use of an active substrate with transistors.
A further force for the manipulation of particles is the force of viscous friction generated by electro-hydrodynamic (EHD) flows, such as electrothermal (ETF) flows or AC electro-osmosis. In N. G. Green, A. Ramos and H. Morgan, J. Phys. D: Appl. Phys. 33 (2000) EHD flows are used to displace particles. For example, the patent No. PCT WO 2004/071668 A1 describes an apparatus for concentrating particles on the electrodes, exploiting the aforesaid electro-hydrodynamic flows.
Other methods are known for the individual manipulation particles in a two-dimensional space. These, however, involve the use of so-called optical or optoelectronic tweezers, i.e., programmable external light sources. The result is a cumbersome and costly system, which is an undesirable characteristic in many applications. In particular A. T. Ohta et al., Tech. Dig. of the Solid-State Sensor, Actuator and Microsystems. Workshop, 216-219, (2004) describes a possible implementation of said techniques.
The limitations of the known art are overcome by the present invention, which enables independent manipulation of a multiplicity of particles in a two-dimensional space, with or without contact depending upon the forces used. The implementation of the method according to the invention does not require the use of electronic circuits or memory elements integrated in the substrate. Different embodiments of the method and apparatus according to the present invention enable manipulation of particles in an n×m two-dimensional array of arbitrary size, with a number of control signals of the order of n+m, or else n, or else even with less than ten control signals, reducing, according to different compromises, the parallelism and flexibility of movement of the particles, and consequently the number of steps to perform a series of displacements (a parameter that is obviously linked to the time of execution).
Even though the methods of the invention can be conducted with substrates without transistors, it is possible, however, to benefit from the use of active substrates to reduce the overall dimensions of the individual elements of the array that constitute the apparatus according to the invention as compared to the known art or to reduce the overall number of the external control signals.
In addition to the possibility of manipulation of cells, the present invention teaches how to combine manipulation and detection by integrating said operations on the same substrate or interfacing sensors and actuators made on different substrates depending upon the technology used.